Cleavage of the relatively inert dinitrogen (N2) molecule, with its extremely strong Nxe2x89xa1N triple bond, has represented a major challenge to the development of N2 chemistry. The relatively inert dinitrogen molecule (N2) composes 78% of the Earth""s atmosphere; the development of this molecule""s chemistry is clearly desirable if this immense natural resource is to be utilized optimally. In this regard, the discovery of mild methods for scission of the Nxe2x89xa1N triple bond represents a major challenge.
The Haber-Bosch ammonia synthesis is the premier example of industrial nitrogen fixation. This process reacts hydrogen and nitrogen at high temperatures and pressures, in the presence of an iron catalyst to produce ammonia, according to eq. (1), 
Based on recent ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy studies, the following mechanism has been proposed for the formation of ammonia, whereby the nitrogen can be adsorbed on an iron surface in both the atomic and molecular states.
H2=2H(ad)
N2=N2(ad)(xcex3)=N2(ad)xcex1)=2N(ad)
N(ad)+H(ad)=NH(ad)
NH(ad)+H(ad)=NH2(ad)
xe2x80x83NH2(ad)+H(ad)=NH3(ad)=NH3
Unfortunately, typical operating temperatures and pressures are in the range of 400-550xc2x0 C. and 100-1000 atm, respectively, thus rendering this process extremely dangerous. Additionally, the necessary equipment for this process is very large and expensive. Naturally, the development of processes at lower temperatures and pressures, preferably standard pressure and temperature, would be economically very attractive, and would reduce the danger involved.
Several modifications of the Haber-Bosch process, such as the Kellogg Ammonia Process, the Topsoe Ammonia Process, the ICI AMV Ammonia Process, and the Braun Purifier Process, have attempted to address these concerns, and have succeeded in increasing efficiency while modestly lowering the temperatures and pressures required (350-470xc2x0 C., 70-105 bar). (C. Hooper, in Catalytic Ammonia Synthesis, J. R. Jennings, Ed. Plenum, New York, 1991). However, these processes still operate at very high temperatures and pressures and the equipment involved is still very specialized, large, and expensive. Thus, there is a continuing interest in developing a catalyst system that would operate at standard temperature and pressure.
The metalloenzyme nitrogenase constitutes a unique biological nitrogen-fixing system capable of nitrogen triple bond cleavage at ambient temperatures and pressures. Nitrogenase catalyzes the reduction of molecular nitrogen to ammonia together with the production of dihydrogen under mild conditions, according to eq. (2),
N2+8H++8exe2x88x92------- greater than 2NH3+H2xe2x80x83xe2x80x83(2)
For many years effort has been expounded in an attempt to develop a model system for this unique biological system. The mechanism of binding and reduction in the biological system has remained elusive, however, recently the crystal structure of the active site in nitrogenase was solved. It is believed that the substrate binding and reduction occur at the multimetallic site involved in the FeMo protein, which consists of Mo and Fe atoms bridged by sulfide ligands. (M. K. Chan et al., Science, 260, p. 792 (1993)). Additionally, nitrogenases have also been discovered which contain vanadium in place of molybdenum or only iron as the transition metal component. This suggests that a wide range of transition metals could potentially facilitate reactions of nitrogen in the coordination sphere. Towards this end, studies of the synthesis and reactions of N2 complexes have been of particular interest in this field. In particular, this area emerged as a result of the discovery in 1965 by Allen and Senoff that [Ru(NH3)5]2+ could reversibly coordinate dinitrogen. (A. D. Allen and C. V. Senoff, J. Chem. Soc., Chem. Commun., p. 621, (1965)).
Since the initial discovery of a complex that could reversibly coordinate dinitrogen, a plethora of N2 metal complexes have been isolated and characterized. N2 is able to bond to a variety of metals with a variety of co-ligands. The nature of the bonding in these complexes varies, from end-on bonding in which the Nxe2x89xa1N bond distance is similar to that in gaseous N2 to linear end-on and side bridging to two or more metals. See, George et al. in xe2x80x9cModeling the Nxe2x89xa1N Bond-Cleavage Step in the Reduction of Molecular Nitrogen to Ammoniaxe2x80x9d, Molybdenum Enzymes, Cofactors, and Model Systems, Ch. 23, pp. 363-376 (1993).
Unfortunately, well-characterized synthetic systems capable of splitting N2 have been elusive despite the multitude of known transition-metal complexes containing intact dinitrogen as a ligand. George et al. in xe2x80x9cReduction of Dinitrogen to Ammonia and Hydrazine in Iron(0) and Molybdenum(0) Complexes Containing the N(CH2CH2PPh2)3 Ligandxe2x80x9d (Inorg. Chem. 34:1295-1298 (1995)) describes the reactions of Fe(N2)(NP3) and Mo(N2)2(NP3) with HBr, where NP3 is N(CH2CH2PPh2)3. Very low yields of hydrazine (N2H4) and N2 were reported.
In all these complexes, there is no demonstrable activation of the Nxe2x89xa1N triple bond. Further, the coordination number of the complexing metal is rather high and in all cases is greater than three, indicating that the metal is not in a very activated state. It is therefore desirable to develop a system having an activated nitrogen triple bond to permit product formation under mild conditions.
It is an object of the present invention to provide a process by which soluble, homogeneous metal complexes are capable of catalyzing the formation of ammonia at ambient temperatures and pressures. It is a further object of the present invention to provide a metal complex possessing an activated nitrogen triple bond which can readily undergo reaction with additional reagents. It is a further object of the invention to provide a metal complex capable of activating a nitrogen-nitrogen triple bond. It is a further object of the invention to provide a metal complex capable of activating a variety of small molecules for reaction with additional reagents.
In one aspect of the invention, a process is contemplated by which soluble, metal complexes are capable of effecting the formation of ammonia from dinitrogen. The metal complex comprises a three coordinate, low oxidation state transition metal complex. The metal complex comprises a metal selected from the group consisting of molybdenum, titanium, vanadium, niobium, tungsten, uranium and chromium, and a plurality of ligands coordinated to the metal such that the metal has a coordination number of no more than three, the ligand sufficiently bulky such that dimerization of the compound does not occur and characterized in that it does not undergo readily xcex2-hydrogen elimination or cyclometallation reactions. A metal complex solution is exposed to dinitrogen under substantially atmospheric pressures and preferably at ambient temperatures, to obtain a metal-nitrido complex, whereby the oxidation state of the metal complex increases. The metal of the metal nitrido complex is then reduced in the presence of a hydrogen source, so as to obtain NH3. Preferred embodiments include the use of hydrogen in the presence of a hydrogenation catalyst, or the use of an acid or reducing agent as the hydrogen source.
In another aspect of the invention, a metal compound is provided which is capable of reductive cleavage of the Nxe2x89xa1N triple bond and reaction with other small molecules. The metal compound comprises a metal selected from the group consisting of molybdenum, titanium, vanadium, niobium, chromium, uranium and tungsten. The metal compound also includes a plurality of ligands coordinated to the metal such that the coordination number is no more than three. The ligand is sufficiently bulky that dimerization of the compound does not occur and is characterized in that it is not capable of a xcex2-elimination reaction or cyclometallation reactions.
In another aspect of the invention, a metal compound for use in activation of small molecules comprises:
xe2x80x83M[NR1R2]3,
wherein M is a transition metal; and R1 and R2 are selected from the group consisting of tertiary alkyls, phenyls and substituted phenyls. In a preferred embodiment, R1 is a tertiary alkyl group and R2 is a phenyl or substituted phenyl group.
In preferred embodiments, the metal complex has the structure, Mo(NRAr)3, where R is C(CH3)3 and Ar is 3,5-C6H3(CH3)2, a synthetic three-coordinate molybdenum(III) complex. The formation of an intermediate complex was observed spectroscopically, and its conversion (with Nxe2x89xa1N bond cleavage) to the nitrido molybdenum(VI) product Nxe2x89xa1Mo(NRAr)3 followed first-order kinetics at 30xc2x0 C. The cleavage reaction proceeds by way of the complex (xcexc-N2){Mo[N(R)Ar]3}2, which, according to EXAFS and NMR data, is formulated as a symmetrical bridging dinitrogen complex with a roughly linear MoNNMo core. (Laplaza et al., J. Am. Chem. Soc., 118, 36, p. 8623 (1996))
xe2x80x9cCoordination numberxe2x80x9d, as that number is used herein, is number of atoms directly bonded to the metal of the complex.